Metallic coiled tubing is used in the oil and gas industry for many applications, especially in the drilling and workover areas. Such tubing is manufactured in a variety of ways, for example, using a continuous milling operation that utilizes paired strips of adjoining tube stock cut to appropriate widths in order to later form desired diameters of coiled tubing. The strips are typically joined together by a welding process that causes the metal to change phase into either a molten or liquefied state, and filler metal or a welding wire is then used to form a weld puddle, thereby providing a suitable weld.
These welded strips are then run continuously through an electronic resistance welding (“ERW”) tube mill to produce a “string” of tubing that can be as much as 20,000 feet in length. The welded string is then placed on a large truck that sets up over a well, and the tubing is repeatedly reeled in and out of the well as various fluids and acids are pumped through the tubular housing.
As many as 15-20 strip welds are required in order to form a long string of tubing (e.g., a thousand feet or more) using this method. As the tubing is forced into and out of the well, it is usually coiled and uncoiled around a truck reel, and consequently the wall of the tubing is stressed and becomes fatigued as the tubing is bent and subjected to flexing and/or high internal pressures.
As those of skill in the pertinent arts will readily appreciate, the welded strip joint has always been the weak link in this process. If the joint or weld fails or ruptures, the results can be catastrophic. At minimum, the tubing can break and fall into the well and delay operations; in some cases, the string becomes tangled or jammed within the well and cannot be removed, thereby jeopardizing the viability of the entire well.
Using current manufacturing methods, it is not possible to create a welded joint having the same molecular structure as the parent material, because all previously employed welding techniques require application of a weldment material (such as wire or the like) in order to complete the weld.
A detailed description of one process for producing a length of coiled tubing can be found in U.S. Pat. No. 5,191,911. In the '911 patent, lengths of metal strip stock are successively drawn from a number of supply coils, and then joined end-to-end to form a composite strip that is fed through a tubing mill. The trailing end of one length and the leading end of the next successive length are cut at supplementary angles, one of which is usually an acute angle. The cut edges are then welded together, and the welded areas are finished to remove excess weldment.
This method frequently involves temporarily extending the width of the edges of the strip at the ends of the weld joint by affixing tabs at the ends of the joint. The tabs serve as temporary heat sinks useful for dissipating heat created during the welding process, and also provide a path for the weldment beyond the edges of the joined strips of metal. After the weld is completed, the tabs are removed during a finishing process. Weldment formed on the surface of the weld must then be removed prior to the tubing being rolled.
The weld is subsequently normalized, so that the heat-affected zone adjacent to the weld returns to nearly the same grain structure as that of the rest of the yet unformed strip. At this point, the composite strip is run through a tubing mill to form seam welded tubing.
Turning for a moment to a seemingly unrelated welding concept, friction stir welding (“FSW”) is a solid-state process by which metals or other materials are joined without the use of fusion or filler materials. FSW has been used in the past to join light-weight metals; most commonly, only aluminum and other highly malleable metals have been welded in this manner. Welds created through FSW result from the combination of frictional heating and mechanical deformation, and do not require application of external weldment material such as welding wire. A detailed description of the FSW process may be found in U.S. Pat. No. 5,460,317.
FSW is most often used when the application requires the characteristics of the resulting material to remain as unaltered as possible. In FSW, two pieces of material are butted together and rigidly clamped to prevent the joint faces from being forced apart. To ensure a quality weld, run-on and run-off tabs are used to permit the starting and stopping of a weld beyond the edge of the metal.
A cylindrical rotary tool with an attached probe is rotated and traversed across (and to a partial extent through) the desired joint region. Significant frictional heat is generated during this process, thereby causing the opposed pieces of metal to temporarily enter into a plasticized and deformable condition while apparently still retaining a solid state. As the rotating probe is traversed along the joint line, the newly plasticized portion is spread along the joint. When the probe is removed, the plasticized region quickly cools, thereby joining the two pieces of metal.
Since there is no melting of an associated weld wire, the heat-affected zone of a friction stir weld is practically eliminated after the process has been completed. This contrasts with the prior art methodology of the '911 patent, in which the weldment-filled, heat-affected zone often comprises the failure point for a given string. Also, since with friction stir welding there is no need for filler wire, there is never any corresponding chemical discontinuity as is associated with the prior art. In short, the hardness variation across a FSW weld is very uniform, thereby eliminating the need to post-heat-treat, as is frequently required with ordinary welding.
To date, however, small edge defects (e.g., minor but noticeable deformations) have frequently been observed after friction stir welding, especially on the advancing side of the tool when welding across a run-on tab, as well as on the retreating side of the tool when traversing a run-off tab, after the tool is rotated across the desired joint region.
These defects are created when the FSW tool traverses the edge of a metal sheet and the flow direction of the tool pulls neighboring material into the structure. Through prior unsuccessful attempts to cure this problem, those of skill in the art have learned that the defect size can be reduced, though not eliminated, through various adjustments in the weld parameters.
To completely eliminate the defects, however, a repair procedure is required to that will consume the deformations. While in many applications of FSW, the minor defects are of little to no consequence, in applications such as in the oil and gas industry, it is extremely important that there are virtually no defects in the finished tubing, as high-temperature, high-pressure fluid flow and other mechanical stresses will eventually result in a localized damaging effect that can ultimately destroy the integrity of associated tubulars.
There is, therefore, a long-felt but unmet need for coiled tubing manufacturing methods that admit to production of continuous strings of tubing, while avoiding the edge deformation and excess weldment issues of the prior art.